Omnidirectional vertically polarized antenna



Oct. 13, 1964 G. J. ADAMS OMNIDIRECTIONAL VERTICALLY POLARIZED ANTENNA Filed sept. 13, 1960 2 Sheets-Sheet 1 .lllllll lili!!! FIGB INVENTOR.

GERALD J. ADAMS BY 049x449,

ATTORNEYS Oct. 13, 1964 G. J. ADAMS OMNIDIRECTIONAL' VERTICALLY POLARIZED ANTENNA 2 Sheets-Sheet 2 Filed Sept. 13, 1960 FIG. 5

INVENTOR.

GERALD J. ADAMS BY ON-ww @www A'IORNEYS United States Patent C) 3,153,239 OMNIDIRECTINAL VERTICALLY PGLARIZED ANTENNA Gerald J. Adams, Cambridge, Mass., assignor to Adams- Russell Co., Inc., Cambridge, Mass., a corporation of Massachusetts Filed Sept. 13, 1960, Ser. No. 55,796 3 Claims. (Cl. 343-791) The present invention relates to high frequency antennas and, in one particular aspect, to omnidirectional antenna arrays of novel and improved compacted construction which promotes electrical efficiency and power transmitting capacity.

As is well understood in the art, the designs of devices which radiate and collect electromagnetic energy are very basically influenced by a number of important factors which tend to be irreconcilable in many cases. Among these are: the critical .behavior of the antenna as a circuit element which must be well matched to its associated transmission line and the surrounding propagation medium over a wide range of frequencies; the need for prescribed directionality in radiation and absorption of energy; the tendencies toward electrical and thermal breakdown under high-power operating conditions; and the need for minimum bulk together with ample mechanical strength under severe ambient environmental conditions. Where high gain is sought through the technique of stacking, the resulting weight and size alone pose mounting difliculties even where the installation may be relatively stationary, and the problems are, of course, compounded where the installation is mobile and must be light and miniaturized while at the same time withstanding vigorous shock, vibration, and variable aerodynamic loadings. Moreover, the mechanical supports required in the stacking of antennas also tend to interfere with the desired radiation patterns, particularly in omnidirectional arrays. Electrically, standing wave ratios and mutual impedance characteristics are likely to be poor when it is sought to consolidate the stacked elements into a compact assembly, and power-handling capabilities are also reduced, in part because the increased difficulty of dissipating heat as the assembly is made more compact, and in part because of the susceptibility to breakdowns where the standing wave ratios are high.

Accordingly, it is one of the objects of the present invention to provide an improved antenna array of stacked elements which exhibits highly compacted bulk and minimizes electrical standing wave ratios.

An additional object is to provide an omnidirectional antenna of light weight, small and symmetrical configuration, and high electrical ethciency, and which possesses mechanical nicety lending itself to low-cost manufacture.

Further, it is an object to provide a compact stacked omnidirectional collinear antenna of high electrical efiiciency and power-handling capacity, the electrical characteristics of which also facilitate its mounting.

By way of a summary account of practice of this invention in one of its aspects, I provide a collinear stack of two (or a geometrical progression of two in which two is the constant multiplier) spaced cylindrical folded-back pairs of radiating elements which are approximately onehalf wavelength in overall length near the middle of a broad frequency range of coverage, each member of the pair being approximately a quarter-wavelength long near the mid-frequency, and the pairs of radiators being fed at their centers by circumferential slots and, generally, in phase with one another. The transmission line which couples the array of radiating elements with a translation device, such as a transmitter, comprises a single coaxial cable disposed centrally of the axial array and hav- Patented Oct. 13, 1964 ing its outer conductor formed as a rigid, tubular mounting unit with integral flange provisions for securing it to a supporting tower, aircraft frame, or the like. This cable has a feed terminus in a peripheral annular slot around its outer conductor, and the latter slot is in turn coupled with the circumferential slots of the radiating elements by way of a divided coaxial path such that the pairs of radiating elements are in series with one another. Importantly, outer ends of the quarter-wave cylindrical elements, remote from the central circumferential feed slots, are terminated in a high reactive impedance conveniently presented by the coaxial line section approximately a quarter wavelength long at mid frequency and formed between the interior of each radiating cylinder and the exterior of the coaxial feed lines, both of which are present for other purposes, whereby the cylindrical radiating sections are advantageously isolated from one another `to improve mutual impedance characteristics and to suppress troublesome standing waves. Hollow cylindrical dielectric sleeves filling the needed divided coaxial pathways develop a solid and ruggedized assembly of only small outer diameter, and a heat-dissipation unit at one end of the array aids in increasing the power-handling capacity of the antenna. In the latter connection, the suppressed standing wave ratio and the high reactive impedance presented by a stub formed as a heat sink both act to increase the power which can safely be propagated through the array.

The subject matter regarded as my invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both as to organization and mode of operation, and further in relation to objects and advantages thereof, this invention may best be understood through reference to the following description taken in connection with the accompanying drawings, wherein:

FIGURE 1 is a longitudinally cross-sectioned View of an omnidirectional antenna array constructed in accordance with these teachings;

FIGURE 2 presents a pictorial illustration of the FIG- URE l array;

FIGURE 3 is a sectioned pictorial representation of part of a two-bay antenna stack, together with superimposed electric lield lines;

FIGURE 4 provides a cut-away pictorial view of an eight-bay stationary collinear array in which this invention is practiced; and

FIGURE 5 illustrates schematically the structure and electric eld patterns of the array shown in FIGURE 4.

The antenna depicted in FIGURES 1 and 2 is a two-bay collinear array which develops an omnidirectional vertically-polarized field pattern about its central longitudinal axis 6--6. Such an assembly is particularly useful in a mobile installation, and may be mounted by way of its base flange 7 with the substantially cylindrical dome 8 projecting downwardly from beneath an aircraft fuselage, for example. In such applications some measure of vertical-plane elevation of the field pattern may -be desirable, and the two stacked antenna sections 9 and 10 which are incorporated within the small thin dome facilitate this for reasons which are explained later herein. Exterior surfaces of the dome are formed by the protective hollow cylindrical sleeve 11, which is an electrical insulator and is transparent to electromagnetic radiation, and by a substantially hemispherical pinned cap 12. A smooth-surfaced iberglass and epoxy resin sleeve, and an anodized aluminum cap, are examples of satisfactory dome member constructions which yield the wanted characteristics. On the opposite side of the conductive mounting flange 7 a high-frequency coaxial cable connector 13 serves to couple the radiating sections with a suitable coaxial transmission line and translating device, such as a high-power transmitter (not illustrated). This coupling occurs through the centermost conductor 14 and its surrounding tubular conductor 15, which together form a short central coaxial transmission line having an impedance properly matched with the impedance characteristics coupled into the connector 13 from external equipment.

Because of certain unique electrical characteristics of the antenna construction, the external conductor 15 of the central coaxial line may itself advantageously be constructed to provide a major part of the mechanical support for the dome and stacked radiating sections on the mounting flange and ground-plane element 7. For this reason, the tubular conductor 15 is fashioned of heavy-gauge stock, with an integral peripheral flange 16 and a thickened annular shoulder section 17 being machined near one end. Flange 16 is recessed into the boss 18 of the mounting flange 7 and is rigidly afiixed to it by bolts 19. For a short distance, the surfaces of the central opening 2t) in boss 18 serve as outer coaxial transmission line surfaces in cooperation with the central rod 14. At its outermost end, the tubular conductor 15 terminates in an annular peripheral slot 21 formed between it and the spaced end of an axially-aligned tubular conductor 22. The latter conductor is bridged at the opposite end 23 and is there firmly secured to the end 14a of center conductor 14 with which it forms a high-frequency stub refiecting a high impedance at the site of slot 21. Solid dielectric filler, in the form of a sleeve 24, occupies the annular space within the stub, and the length of the stub is selected such that it exhibits high impedance in the manner of a shorted quarter-wave line at the frequencies involved, modified, of course, by the difference in length dictated for such impedance-matching purposes as may need to be satisfied.

When high frequency electrical energy is applied to the coupling 13, it travels by way of the inner coaxial line to the aforementioned slot 21. Thence, it is propagated directly into the coaxial lines formed between a surrounding hollow cylindrical conductive post 25 and each of the exteriors of the input coaxial conductor 15 and the stub coaxial conductor 22. The field emitted from slot 21 divides in two, one half travelling to an output slot 26 at an end of the coaxial line formed between conductors 15 and 25, and the other half travelling to another output slot 27 at an end of the coaxial line formed between conductors 22 and 25. Solid dielectric spacers 28 and 29 occupy the annular gaps appearing in these lines, respectively, thereby providing a rigid mechanical mounting for the coaxial post 25 on the assembly as thus far described. From each of the spaced peripheral output slots 26 and 27, the field appearing there is propagated outwardly to the cylindrical radiating elements 30 and 31, cooperating with peripheral slot 26, and sections 32 and 33 cooperating with peripheral slot 27. As is well-known, the optimum length of cylindrical radiating elements having an appreciable diameter is in fact less than the freespace quarter-wavelength of the energy being radiated, and while quarter and half-wavelengths are referred to herein it should be understood that these are the customarily-identified lengths although in practice the actual optimum values may therefore differ somewhat from the theoretical mathematical values. Similarly, where reference is made to quarter-wave shorted stub or choke elements, it should be understood that the actual lengths may differ somewhat from the mathematically-calculated values for the wavelengths involved, particularly where some degree of reactive impedance may also be desirable for matching purposes, although the generally high irnpedance of such elements is what is principally of interest. The approximate wavelengths at substantially the mid frequency of a broad band of desired coverage are also intended in each instance.

The annular output slots 26 and 27 are intentionally made of an appreciable radial width, as governed by annular flanges such as those identified by numeral 34 which extend radially outward from ends of center post 25 and from conductors 15 and 22. At their radially outermost ends, these anges mount the quarter-wave elements 30- 33 and thereby form the desired half-wave radiator terminations for the spaced slots. The two radiator sections 9 and 10 must be isolated from one another to operate most effectively and to exhibit optimum mutual impedance characteristics, but at the same time it is important that the antenna array be of the smallest possible size. These interests are reconciled through introduction of quarterwave shorted coaxial lines exhibiting high reactive impedances at the outer ends of the quarter-wave radiating elements. One such choke or stub element, 35, is formed by the inner surfaces of section 31 and the opposite outer surfaces of post 25, as shorted by the flange 34 between them. Another, 36, is formed between the corresponding inner and outer surfaces of section 32 and post 25. Choke sections 35 and 36 are disposed at the sites of adjacent end of `the collinear radiation sections 9 and 10 and are thereby effective to isolate them. This involves a physical axial spacing of the adjacent radiating elements, 31 and 32, also, which is conveniently insured by dimensioning the annular post 25 such that it is longer than onehalf a wavelength at the frequencies involved. The resulting axial spacing between adjacent collinear antenna sections 9 and 10 is also in the nature of a peripheral slot, and this slot also cooperates with the quarter-wavelength radiating elements and the output slots to produce the complex effects which yield the desired radiations from the stack. This axial spacing need not be large, whereby the overall length of the array is minimized.

Importantly, a comparable annular choke element, 37, is developed by exploiting the quarter-wavelength inner surface of dipole section 33 and the exterior of shoulder 17 of coaxial transmission line conductor 15 for this purpose. Again the associated flange about the output slot, 26, provides the needed shorting. Choke element 37 offers a high reactive impedance which electrically isolates the array from the mounting flange 7 and from the aircraft fuselage or other support by which the array is carried. Conductive fiange 7 forms a ground plane, which is extended by any conductive aircraft surface on which it is positioned, and thereby multiplies the gain of the antenna array. This ground plane, in combination with the choke element 37, permits the diameter of the radiating elements to be significantly smaller than would be the case with an array lacking the choke element and having the same standing wave ratio or degree or mismatch. Broad band characteristics and a high degree of miniaturization are important advantages promoted by this construction.

A fourth comparable annular choke element 38, at the outer free end of the array, is formed by inner surfaces of element 30 and by the outer surface of a cylindrical extension 39 of tubular conductor 22. Extension 39 terminates in an annular ange 40 of conductive material, and is hollow and perforated to receive a desiccant material such as silica gel which absorbs moisture appearing in the sealed array. Cap 12 is of aluminum, protectively anodized, and is in part supported by way of its central pin 41 and a mounting bushing 42 set into extension 39.

With the constructional features described, the collinear array possesses a very low voltage standing wave ratio and is operative with high gain and efiiciency over a wide range of frequencies. The power-handling capacity, which is improved by virtue of the low standing wave ratio, is further augmented by a heat dissipation unit 43 at the mounting base. This unit takes advantage of the known high-impedance characteristics of a quarterwave shorted stub in making good thermal contact with both conductors of the central coaxial transmission line while avoiding an electrical short-circuiting at the high frequencies at which the antenna is used. A copper bar 44 serves as the center conductor of the coaxial heatdissipating stub, one end being in intimate engagement with the coaxial transmission line center conductor 14 and the other end, one quarter-wavelength distant, being connected in good electrical and thermal connection with the hollowed casting 45 by way of a bolt 46. Hollowed recess 47 in the casting 45, which is part of the outer coaxial conductor of the feed line, is proportioned to yield the desired high-impedance stub characteristics. Bar 44 and casting 45 dissipate unwanted thermal energy of the feed line and antenna assembly through the flange 7 and, in turn, through the structure (not illustrated) upon which it is supported.

The conditions which produce in-phase excitations of both dipoles are exhibited in connection with the generally similar structure of FIGURE 3, wherein the explanatory linework bearing arrows indicates instantaneous electric eld lines. For convenience, these features which correspond to features of the assembly of FIGURES 1 and 2 are identified by the same reference characters, with distinguishing single-prime accents added. The radiated fields are of course omnidirectional, circularly about the collinear array axis 6-6. Elevation in the axial direction is achieved, as desired, by slightly altering the phase relationships of the radiated fields. This is accomplished in a simple manner by making the lengths of feed between the common slot 21' and each of the output slots appropriately different. Where the length from slot 21 to output slot 26 is the larger, for example, the field from slot 26 and section 10 is delayed in phase relative to that of section 9' and the resulting beam is caused to have a desired tilt away from flange 7.

Another construction in which these teachings are practiced appears in a partly sectioned view in FIGURE 4 and schematically in a single-line layout in FIGURE 5. Omnidirectionality is assured about the vertical axis 50-50, and a slight beam tilting upwardly of the base mounting flange 51 is secured by a controlled phasing of output radiation form the eight isolated collinear radiating units 52-59. Reference characters applied to the schematic illustration of FIGURE 5 are distinguished by singleprime accents from those which make the corresponding identifications in FIGURE 4. This array is intended for flange-mounting upon a stationary support, such as a tower or building, and therefore carries needed warning beacon lamp units 60 at the top end. Conveniently, these units receive their low-frequency power excitation from a single pair of power leads 61 and 62 connected near the base to the center high-frequency coaxial conductor 63 and its insulated inner lead wire 64, respectively, through a connector 65 and a high-impedance quarter-wave shorted stub 66. Conductor 63 is electrically connected with conductive member 67 near the top of the assembly, such that these two conductors, 67 and 64, can excite the beacon lamp units with low-frequency power, without introducing external leads which would intercept and interfere with the omnidirectional eld pattern of the antenna stack. In similar fashion, an internal blower 68 can be energized to circulate entrapped gases in the sealed antenna for heat-dissipation purposes, without disturbing the critical high-frequency circuitry of the assembly.

The coaxial transmission line which couples energy between the vertical collinear array and the associated translating equipment, such as a high-frequency transmitter, includes the aforementioned central conductor 63 and a proximately-surrounding tubular conductor 69 which, at its lower flanged end, is of a relatively large diameter and thickened cross-section permitting it to carry substantially all of the static and dynamic loading of the array. This coaxial feed line terminates in an annular peripheral slot 70 substantially mid-way along the antenna stack. There, the center conductor 63' is atlixed to the shaped upper conductive tubular member 67', and the instantaneous electric field lines, shown by the unnumbered linework carrying arrows, illustrate the manner in which transmitted high-frequency energy is propagated radially outwardly. At this site, the energy is divided substantially in two and is propagated upwardly, to annular slot 71', between a surrounding spaced tubular conductor 72' and the shaped member 67', and downwardly, to annular slot 73', between conductor 72' and the exterior of the upper part of outer coaxial feed line 69. In turn, the energy propagated radially outwardly from inner slot 71' divides substantially in two, one half travelling upwardly, to a further intermediate slot 74', between the exterior of shaped conductor 67' and the interior of a surrounding radially-spaced tubular conductor 75', and the other half travelling downwardly, to intermediate slot 76', between the exterior of tubular conductor 72 and the interior of conductor 75'. In like manner, the energy from inner slot 73' divides such that substantially one-half travels upwardly to an intermediate slot 77 between the exterior of conductor 72' and the interior of a surrounding spaced tubular conductor 78', and downwardly to an intermediate slot 79', between the shaped exterior of skirt 80" of coaxial feed line 69 and the interior of conductor 78. Each of the intermediate slots divides lits output in the same manner, with the result that the eight output slots 81'- 88 are fed with substantially equal amounts of power, and with their electric field lines substantially in phase. The last division entails the use of tubular conductors 89-95', each of which is folded back such that it is terminated in substantially quarter-wave radiating elements, and such that these radiating elements are terminated in substantially quarter-wave shorted high-impedance chokes. Added chokes, 52a and 59h', are provided at the axial extremeties of the array to isolate the top beacon assembly and base support assembly, respectively, from the radiating array and, thereby, to insure that the desired field pattern is not distored. As in the case of the arrayl of FIGURES 1-3, the adjacent sections are slightly spaced in the axial direction, and the endmost chokes are also spaced from the top and bottom assemblies. Although for clarity the needed dielectric spacers are not all illustrated for the support of various tubular conductors which are not otherwise suspended, it will be understood that such spacers may comprise sleeves or bushings of conventional dielectric material, such as bushings 96 and 97, for example. Preferably, the lengths of the passageways to the lower output slots and dipoles are made slightly longer than the others, to produce slight phase displacements which cause some upward beam tilting. This is achieved by appropriate mechanical dimensioning of the passageways. Outer cylindrical sleeve 98 is transparent to electromagnetic radiation and protectively encloses the dipole array. A dry gaseous filling within the sealed enclosure aids in protecting the components and serves as a convection medium for dissipating thermal losses.

Those skilled in the art will recognize that the preferred specific embodiments selected for detailed discussion may be altered in many respects in other practices of this invention. An example is found in the use of elements shaped other than in the cylindrical form illustrated. Accordingly, it should be understood that the embodiments disclosed herein are intended to be of a descriptive rather than a limiting nature and that various changes, combinations, substitutions or modifications may be effected without departing in spirit or scope from this invention its broader aspects.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A stacked antenna array comprising collinear axially-spaced antenna sections each of which is formed by two collinear tubular conductive elements extending from opposite sides of a transverse slot therebetween and each having an axial length substantially equal to a quarter wavelength near the mid frequency of a predetermined band of high operating frequencies, tubular conductor means inwardly coaxial with and radially spaced from said tubular elements of said collinear antenna sections which are adjacent one another, means electrically connecting said adjacent elements of said adjacent antenna sections with said tubular conductor means at the sites of the slots and forming substantially quarter-wavelength coaxial stubs exhibiting high impedances at the adjacent ends of said adjacent tubular elements of said antenna sections which suppresses ow of high-frequency energy between said slots, a coaxial transmission line disposed radially inwardly of and coaxially with said tubular elements and said conductor means, means guiding substantially equal amounts of high-frequency energy between said coaxial transmission line and each of said slots and preserving simultaneously substantially the same phase of excitation of the antenna sections and one phase of excitation of the junction between said coaxial line and guiding means, dielectric means rigidly supporting said conductor means on said coaxial transmission line, the outer conductor of said coaxial transmission line being rigid and thereby supporting loads of the antenna array, mounting means ixed with said outer conductor beyond one axial end of the collinear array of said antenna sections, and different means at each axial end of said collinear array each having electrically conductive surfaces coaxial with the nearest tubular element of the array and electrically connected with the nearest tubular element at the site of the nearest slot to form a substantially quarter-wavelength high-impedance coaxial stub, whereby ow of high-frequency energy beyond the axial extremities of said antenna sections in the array and to said mounting means is suppressed.

2. A stacked antenna array as set forth in claim 1 wherein said mounting means includes an electricallyand thermally-conductive flange having an opening therethrough forming an extension of said outer conductor of said coaxial line, said mounting means having a substantially planar surface which extends substantially perpendicular to said collinear antenna sections for distances greater than the radial dimensions of said antenna sections and having a recess therein disposed on the side of said surface opposite that on which said antenna sections are located, said recess being closed at one end and communicating with said opening, an electricallyand thermally-conductive bar connected with the center conductor of said coaxial line and extending through said recess into connection with said flange at the closed end of said recess, said bar and the surfaces of said flange bordering said recess forming a heat sink and a substantially quarterwave coaxial stub exhibiting high impedance to said coaxial line at said operating frequencies.

3. A stacked antenna array as set forth in claim 1 wherein said mounting means includes an electrically conductive flange having a substantially planar conductive surface transverse to the central axis of said collinear array and axially spaced from the end of the tubular element nearest said one end of said array, said surface of said flange extending laterally of said axis by distances greater than the radial dimensions of said antenna sections and forming an isloated ground plane surface for said array.

References Cited in the le of this patent UNITED STATES PATENTS 

1. A STACKED ANTENNA ARRAY COMPRISING COLLINEAR AXIALLY-SPACED ANTENNA SECTIONS EACH OF WHICH IS FORMED BY TWO COLLINEAR TUBULAR CONDUCTIVE ELEMENTS EXTENDING FROM OPPOSITE SIDES OF A TRANSVERSE SLOT THEREBETWEEN AND EACH HAVING AN AXIAL LENGTH SUBSTANTIALLY EQUAL TO A QUARTER WAVELENGTH NEAR THE MID FREQUENCY OF A PREDETERMINED BAND OF HIGH OPERATING FREQUENCIES, TUBULAR CONDUCTOR MEANS INWARDLY COAXIAL WITH AND RADIALLY SPACED FROM SAID TUBULAR ELEMENTS OF SAID COLLINEAR ANTENNA SECTIONS WHICH ARE ADJACENT ONE ANOTHER, MEANS ELECTRICALLY CONNECTING SAID ADJACENT ELEMENTS OF SAID ADJACENT ANTENNA SECTIONS WITH SAID TUBULAR CONDUCTOR MEANS AT THE SITES OF THE SLOTS AND FORMING SUBSTANTIALLY QUARTER-WAVELENGTH COAXIAL STUBS EXHIBITING HIGH IMPEDANCES AT THE ADJACENT ENDS OF SAID ADJACENT TUBULAR ELEMENTS OF SAID ANTENNA SECTIONS WHICH SUPPRESSES FLOW OF HIGH-FREQUENCY ENERGY BETWEEN SAID SLOTS, A COAXIAL TRANSMISSION LINE DISPOSED RADIALLY INWARDLY OF AND COAXIALLY WITH SAID TUBULAR ELEMENTS AND SAID CONDUCTOR MEANS, MEANS GUIDING SUBSTANTIALLY EQUAL AMOUNTS OF HIGH-FREQUENCY ENERGY BETWEEN SAID COAXIAL TRANSMISSION LINE AND EACH OF SAID SLOTS AND PRESERVING SIMULTANEOUSLY SUBSTANTIALLY THE SAME PHASE OF EXCITATION OF THE ANTENNA SECTIONS AND ONE PHASE OF EXCITATION OF THE JUNCTION BETWEEN SAID COAXIAL LINE AND GUIDING MEANS, DIELECTRIC MEANS RIGIDLY SUPPORTING SAID CONDUCTOR MEANS ON SAID COAXIAL TRANSMISSION LINE, THE OUTER CONDUCTOR OF SAID COAXIAL TRANSMISSION LINE BEING RIGID AND THEREBY SUPPORTING LOADS OF THE ANTENNA ARRAY, MOUNTING MEANS FIXED WITH SAID OUTER CONDUCTOR BEYOND ONE AXIAL END OF THE COLLINEAR ARRAY OF SAID ANTENNA SECTIONS, AND DIFFERENT MEANS AT EACH AXIAL END OF SAID COLLINEAR ARRAY EACH HAVING ELECTRICALLY CONDUCTIVE SURFACES COAXIAL WITH THE NEAREST TUBULAR ELEMENT OF THE ARRAY AND ELECTRICALLY CONNECTED WITH THE NEAREST TUBULAR ELEMENT AT THE SITE OF THE NEAREST SLOT TO FORM A SUBSTANTIALLY QUARTER-WAVELENGTH HIGH-IMPEDANCE COAXIAL STUB, WHEREBY FLOW OF HIGH-FREQUENCY ENERGY BEYOND THE AXIAL EXTREMITIES OF SAID ANTENNA SECTIONS IN THE ARRAY AND TO SAID MOUNTING MEANS IS SUPPRESSED. 